cmd/go: document using 'go list' to download modules needed to build packages #38031
Description
go list -test ./... can be used to download the modules needed to build tests of the packages matched by ./.... This is usually a smaller set of modules than go mod download all (or equivalent go mod download) would select, so it's a useful command when building small Docker images.
This should be documented as a FAQ on https://github.com/golang/go/wiki/Modules.
Related #36460: fewer modules will be downloaded by go mod download all when lazy module loading is implemented.